This invention relates to the production of transparent, electrically conductive and infrared-radiation-reflecting indium-tin-oxide layers (ITO layers) using the conventional dipping method. In this process, glass panes are dipped into a solution of hydrolyzable compounds of indium and tin, drawn uniformly out into a steam-containing atmosphere, dried, and hardened under the effect of a reducing gas atmosphere.
ITO layers are of considerable interest on account of the combination of properties they provide, i.e., high electric conductivity, high IR reflection with high solar energy transmission, high transmission in the visible range, ready etching capability using acids, good environmental stability, good scratch resistance, and good adhesion to glass, even in comparison with other layers such as tin oxide, cadmium stannate and thin copper, silver or gold layers between dielectric layers. These layers are inferior to ITO layers with respect to their combination of properties.
Unilateral ITO layers on thin glass, produced by the vacuum method, have become quite extensively popular in the display field (LCD and others). The combination of high solar energy transmission with simultaneously high IR reflection of long wavelength IR radiation is a very desirable property in connection with coatings in the glazing of buildings (insulating glass windows). These properties allow the window to become a passive solar collector. High transmission is desired from the viewpoint of light technology, and almost ideal requirement profiles are met by ITO layers. Such layers do not exist commercially. Other fields of application include solar cells (ITO, CdS, CuS), photocells (ITO, PbS), optical filters, electric heating, antifogging devices and methods, and many others. Those skilled in the art are familiar with the great significance of ITO layers, especially when considering environmental stability as well.
In accordance with the known state of the art, ITO layers are produced by vacuum methods, including sputtering. The expenditure in apparatus is considerable, and the application rate is relatively slow. Therefore, thus far, there exists no large area surface coating method extending over several square meters.
ITO layers are also applied by the spraying and CVD techniques. Layers applied in this way do not exhibit the adequate uniformity required for a window. This is essentially due to the occurrence of various interference colors because of varying thickness. Heretofore, this process has not enabled the production of large-surface coatings, either.
Highly homogeneous layers of unifom thickness can be applied to large glass panes by the dipping method. In this process, the pane is dipped into a solution of hydrolyzable metal compounds, such as, for example, silicic acid esters in alcohol, drawn out at uniform speed, then air-dried, and hardened at 400.degree.-500.degree. C. and thus transformed into a transparent SiO.sub.2 layer.
Although layers can be produced with a plurality of various oxides (H. Schroeder, Oxide Layers Deposited from Organic Solutions, Physics of Thin Films, vol. 5, 1969, Academic Press Inc., New York) and a method exists for the production of defined multicomponent oxides (H. Dislich, Angew. Chem. Internat. Ed. vol. 10, 1971, No. 6: 363-370), heretofore, no solution has been provided for the problem of producing high-conductivity and highly IR-reflecting ITO layers in a dipping procedure, although this has been recognized as a desirable aim.
For example, in U.S. Pat. No. 4,252,841, a dipping process was attempted. In U.S. Pat. No. 4,268,539 as well as British Pat. No. 2,056,433, it was tried by utilizing other methods. Surface resistances of at best only 500 .OMEGA./.quadrature. have been obtained. To provide a useful advance in the art, however, the range of a defined 20-30 .OMEGA./.quadrature. for windows and 10-500 .OMEGA./.quadrature. for displays is necessary. Moreover, for practical exploitability, a parcel of properties must be attained as per the following target data:
______________________________________ Property Windows Displays ______________________________________ Surface resistance (.OMEGA./.quadrature.) 20-30 10-500 IR Reflection at 9.5 .mu.m (%) 70-80 -- Residual reflection in -- &lt;10 visible range (%) Transmission in visible range (%) &gt;80 &gt;80 Color reproduction neutral Yes Yes Hardness, brass-iron-proof Yes Yes Homogeneous layer thickness Yes Yes Smooth layers Yes Yes Stable against usual cleansers Yes Yes Stable against environment, Yes -- including sun Permits single etching -- Yes ______________________________________
According to the state of the art, these requirement profiles are not achieved in the dipping process suitable for large-surface coating. Not even substantial steps toward this direction have become known. However, the dipping method would be the choice selected because it possesses a high degree of reproducibility of properties, and above all because both sides of a glass pane are coated simultaneously and without additional expenditure whereby functional efficiency is considerably enhanced. Thus, in case of displays, in the so-called dual cells, the central pane with its conductive layer on both sides is utilized, and in case of windows, functional efficiency is considerably increased as can be seen from the following data. For an insulating glass window made up of two 6 mm float glass panes (one coated bilaterally) at a spacing of 12 mm, interspace filled with argon, the following values are obtained
______________________________________ light transmission L = 83% total energy transmission G = 74% heat transfer coefficient k = 1.5 W/m.sup.2 K.sup. o ______________________________________
The best values for commercially available systems are presently L=69%, G=60%, k=1.5 W/m.sup.2 K.
The advance in the art attainable with the use of ITO layers is especially striking when employing a window as a passive solar collector. Additionally, the ITO layers produced by the dipping method show long-term stability against weathering, even if one of the two layers faces the outside.